In high-speed reproduction machines, such as electrostatographic copiers and printers, a photoconductive member (or photoreceptor) is charged to a uniform potential and then a light image of an original document is exposed onto a photoconductive surface, either directly or via a digital image driven laser. Exposing the charged photoreceptor to a light image discharges the photoconductive surface thereof in areas corresponding to non-image areas in the original document while maintaining the charge on the image areas to create an electrostatic latent image of the original document on the photoconductive surface of the photoreceptor. A developer material is then brought into contact with the surface of the photoconductive member to transform the latent image into a visible reproduction. The developer material includes toner particles with an electrical polarity opposite that of the photoconductive member, causing them to be naturally drawn to it. A blank print substrate such as a sheet of paper is brought into contact with the photoconductive member and the toner materials are transferred to it by electrostatic charging of the substrate. The substrate is subsequently heated and pressed to permanently bond the reproduced image to the substrate, thus producing a hard print reproduction of the original document or image. Thereafter, the photoconductive member is cleaned and reused for subsequent print production.
The process of transferring charged toner particles from an image bearing member, such as the photoreceptive member, to an image support substrate, such as a print sheet, is accomp lished at a transfer station. In a conventional electrostatographic machine, transfer is achieved by transporting an image support substrate into the area of the transfer station where electrostatic force fields sufficient to overcome the forces holding the toner particles to the photoconductive surface are applied to the substrate to attract and transfer the toner particles to the image support substrate. In general, such electrostatic force fields are generated via electrostatic induction using a corona generating device. The reverse side of the print sheet is exposed to a corona discharge while the front of the print sheet is placed in direct contact with the developed toner image on the photoconductive surface. The corona discharge generates ions having a polarity opposite that of the toner particles, thereby electrostatically attracting and transferring the toner particles from the photoreceptive image bearing member to the print sheet.
The interface between the image bearing surface and the print sheet, however, is not always optimal. In particular, non-flat or uneven image support substrates, such as copy sheets that have been mishandled, paper that has been left exposed to the environment, or substrates that have previously passed through a fixing operation (for example, heat and/or pressure fusing) often tend to yield imperfect contact with the photoconductive surface. Some printing applications require imaging onto high quality papers having surface textures which prevent intimate contact of the paper with the developed toner images. In duplex printing systems, even initially flat paper can become cockled or wrinkled as a result of paper transport and/or the first side fusing step. Also, color images can contain areas in which intimate contact of toner with paper during the transfer step is prevented due to adjacent areas of high toner pile heights. The lack of uniform intimate contact between the belt and the copy sheet in these situations can result in spaces or air gaps between the developed toner powder image on the selectively charged imaging surface and the copy substrate. When spaces or gaps exist between the developed image and the copy substrate, various problems may result. For example, there is a tendency for toner not to transfer across gaps, causing variable transfer efficiency and, under extreme circumstances, creating areas of low toner transfer or even no transfer, resulting in a phenomenon known as image transfer deletion.
In order to minimize transfer deletions, transfer assist blades (TABs) have been utilized to press the back of the copy substrate against the imaged area of the charged imaging surface. The transfer assist blade is typically moved from a non-operative position spaced from the copy substrate, to an operative position in contact with the copy substrate. A mechanism supporting the TAB is operable to press the TAB against the copy sheet with a typically pre-determined force sufficient to press the copy substrate into contact with the developed image on the photoconductive or other charged imaging surface in order to substantially eliminate any spaces therebetween during the transfer process.
While the transfer assist apparatus of the type described above may be used to improve transfer efficiency, it may also induce copy quality defects in the lead edge area of the copy sheet. For instance, when the developed toner image extends to the lead edge of the imaged area on the photoreceptor, there may be a loss of toner electrostatic tack force in the lead edge region. As a result, drag force on the image substrate caused by pressing the TAB onto the substrate may be greater than the tack force between the substrate and the photoreceptor, which, in turn, generates a velocity mismatch between the copy sheet and the photoreceptor, manifesting itself as a smeared image on the lead edge of the copy sheet. This lead edge image defect is unacceptable in most high speed environments where customers demand lead edge to trail edge copy quality as the electrostatographic printing process penetrates further into the offset printing market.
One method that has been utilized to minimize the leading edge smear defect is delaying the engagement of the TAB to allow the electrostatic tacking force to increase by providing a timing delay between sensing of the leading or trailing edge of a copy substrate and actuation of the mechanism that urges the TAB toward the copy substrate. Delaying engagement of the TAB, however, may result in spaces or air gaps between the developed toner powder image on the selectively charged imaging surface at the lead edge resulting in transfer deletions at the lead edge.